Generally, dialkylbenzenes obtained by the dealkylation of benzenes are a mixture of 1,2-, 1,3-, and 1,4-isomers, but a difference in boiling point between these isomers is so small that, in many cases, even rectifying columns having many plates are insufficient to separate these isomers from one another by distillation.
Next, one specific example will be given. Cymene isomers obtained by the alkylation of toluene with propylene have the following boiling points: o-Isomer, 178.3.degree. C.; m-isomer, 175.1.degree. C.; and p-isomer, 177.1.degree. C. A difference in boiling point between m- and p-cymenes, which comes into special question in the cresol manufacturing process, is only 2.degree. C., so that separation of the both by rectification is extremely difficult. In the cresol manufacturing process now in use, therefore, the following procedure is employed: The mixed cymene, without being separated into the isomers, is oxidized as such into a mixed cresol, and thereafter, separation of the cresol isomers is carried out.
As one method to separate the cresol isomers from one another, there is a method in which the cresol mixture is alkylated with isobutylene into a mixture of tert-butyl cresol isomers, the isomers are separated from one another by rectification taking advantage of a large difference in boiling point between them, and then the tertiary butyl group is eliminated to obtain high-purity m- and p-cresols.
As another method to separate the cresol isomers from one another, there is a method in which a mixture of cresol urea isomeric clathrate compounds is separated into the isomers by recrystallization taking advantage of a difference in crstallizability between them, and the separated compounds are decomposed to obtain high-purity m- and p-cresols.
The foregoing both methods are a separation method now in use in industry, but their process is so complicated that a furthermore improvement is desired.
Another specific example will be given below. Diisopropylbenzene obtained by the alkylation of benzene, which is a starting material for 1,3-dihydroxybenzene (resorcinol) and 1,4-dihydroxybenzene (hydroquinone), comprises the isomers having the following boiling points: o-Isomer, 200.degree. C.; m-isomer, 203.2.degree. C.; and p-isomer, 210.3.degree. C. A difference in boiling point between m- and p-diisopropylbenzenes, which comes into special question in the resorcinol and hydroquinone manufacturing process, is 7.degree. C., so that separation of the both by rectification is possible. This method, however, requires rectifying columns having a fairly large number of plates so that it may not always be said to be a separation method of good efficiency.
Instead of these conventional separation methods, there are proposed ones based on a new idea which are intended to selectively dealkylate only the 1,4-dialkyl isomer in the dialkylbenzene, to thereby recover the 1,3-dialkyl isomer (in some cases, 1,2- plus 1,3-dialkyl isomers) as unreacted (Japanese Patent Application (OPI) Nos. 83716/1980 and 83721/1980). (The term "OPI" as used herein refers to a "published unexamined Japanese patent application", hereinafter the same.) This method uses a ZSM type zeolite as a catalyst, and particularly, a ZSM type zeolite catalyst modified with oxides such as MgO, P.sub.2 O.sub.5, etc., dealkylates only the 1,4-dialkyl isomer with a very high selectivity, so that this method is a markedly epoch-making technique.
From the practical point of view, however, this method also has a large defect that, when the alkyl group to be dealkylated has three or more carbon atoms, olefins obtained by the dealkylation are low in purity and percent recovery. For example, Example 10 of Japanese Patent Application (OPI) No. 83716/1980 discloses that m-cymene is obtained in a high purity (96.6%) by dealkylating a mixed cymene (o/m/p=2.16/66.16/31.67) using a steam-treated H-ZSM-5, but the purity of propylene recovered at that time is about 60% in the volatile gas obtained. Similarly, Example 11 of Japanese Patent Application (OPI) No. 83721/1980 discloses that a high-purity m-cymene is obtained by dealkylating a mixed cymene using a similar catalyst, but the purity of propylene recovered at that time is 43%. When these known methods were verified by the present inventors, many kinds of C.sub.2 -C.sub.6 olefins and paraffins were found in the recovered propylene, in addition to propylene, and so it was supposed that the eliminated isopropyl group was subjected to complicated side reactions such as oligomerization, cracking, hydrogenation, etc. Further, from the total carbon content of the C.sub.2 -C.sub.6 volatile gases which was lower than that calculated from the eliminated isopropyl groups, it was supposed that some parts of the latter were changed to heavy components having more than six carbon atoms. Thus, these known methods had not only a defect that the percent recovery of the recovered olefins was very low but also a defect that the purity of the recovered olefins was low and, accordingly, separate olefin-purification equipments were required.
On the other hand, Japanese Patent Application (OPI) No. 103119/1981 discloses that when the reaction is carried out in the presence of an H-ZSM-5 catalyst while feeding a mixed cymene together with aniline or ammonia, the dealkylation proceeds with a high para-selectivity, whereby propylene is recovered in a high purity as 94%. However, this method also had the following defects: Namely, when an actual embodiment to be applied industrially is taken into account, while the recovered toluene and propylene are recycled into the alkylation region, it is necessary to separate the aniline or ammonia from the recovered toluene and propylene since the alkylating catalyst would be deactivated by a base such as the entraining aniline or ammonia, if any. Thus, the process cannot be said as economical one.
The present inventors have already proposed a method using a crystalline zeolite catalyst ion-exchanged with lithium ions, as a method for selective dealkylation at the para-position, which was improved in various defects of the above-described known methods (Japanese Patent Application (OPI) No. 216835/1984).
According to the above-proposed method, the dealkylation occurred selectively at the para-position, and recovered olefins can be obtained in a high percent recovery and a high purity. However, since the higher the degree of ion-exchange, the better the result, the preparation of the catalyst was not easy. On ion-exchanging a crystalline zeolite with lithium ions, it was not so easy to enhance the degree of ion-exchange, probably because the lithium ions were hydrated and their ionic radius was increased, and a special treatment such as a treatment at high temperatures was therefore required. Thus, the method using such a catalyst was not necessarily satisfactory on an industrial scale.
Furthermore, at that time the inventors believed that these specific effects of the lithium ions were characteristic for the lithium ions only, and other alkali metal ions such as sodium ion, potassium ion, rubidium ion, and cesium ion could not show such superior properties. But with a view to finding out a more excellent method for selective dealkylation at the para-position, the present inventors have made further investigations on various ion-exchanged zeolite catalysts and, as a result, found that the selective dealkylation at the para-position can be performed with equal or higher yield and purity of recovered olefins, using the zeolite catalyst ion-exchanged with a specified amount of a specific base ion (other than lithium ion), which can be prepared easily under very mild conditions unlike the catalyst ion-exchanged with lithium ions. The present invention has been thus accomplished.